You're spending an awful lot of time in this letter talking about how Sprint's terminating your phone number affected you.

What you don't mention is why they did it. You call it "a mistake", but that's it. The person getting this letter is probably not already familiar with your situation, and you don't know how detailed or accurate the notes about these incidents are in the records available to them.

If in fact it was a mistake on their part that happened despite you living up to your end of the bargain, you should really stress that. Say what reason they gave when it happened. If they told you it was your fault say what proof you offered that it was their error. Whoever gets this letter is likely to have a default assumption that you did something wrong, twice, and that's why your service got cut off, because that's what happens in an awful lot of cases, and you need to convince them that your situation is different.

If, on the other hand, they had a reason (such as you not paying your bills) to terminate your number according to the contract you signed, then how it affected you is totally irrelevant. They don't provide service to people based on how deserving they are or how much they need that service; they provide service to people based on how they behave in their business dealings with Sprint.

And while I congratulate you on your honesty in acknowledging that not all of the 22 accounts are necessarily be canceled in support of you, the threat isn't really relevant either. Again, if they have wronged you, you should be stressing that rather than the red herrings of how they affected you and how you can affect them; and if you have wronged them, that's your problem.